1. Field of the Invention
The invention relates to fuel injectors wherein air and fuel are mixed before combustion. It has particular application to fuel injectors used for combustors in gas turbine engines.
2. Discussion of Prior Art
Gas turbine engines include an air intake through which air is drawn and compressed by a compressor and thereafter enters a combustor at one or more ports. Fuel is injected into the combustion chamber by means of a fuel injector where it mixed with compressed air from the various inlet ports and burnt. Exhaust gases are passed out of an exhaust nozzle via a turbine which in turn drives drive the compressor. In addition to air flow into the combustion chamber through the air inlet ports, air also enters the combustion chamber via the fuel injector itself. The fuel injector is therefore different from fuel injectors in Diesel engines, for example, in that air is mixed with fuel before entering the combustion chamber. Fuel injectors therefore provide an air/fuel xe2x80x9csprayxe2x80x9d comprising of droplets of fuel atomised in air which enters the combustion chamber.
Conventional combustors take a variety of forms. They generally comprise a combustion chamber in which large quantities of fuel are burnt such that heat is released and the exhaust gases are expanded and accelerated to give a stream of uniformly heated gas. Generally the compressor supplies more air than is needed for complete combustion of the fuel and often the air is divided into two or more streams, one stream introduced at the front of the combustion chamber where it is mixed with fuel to initiate and support combustion along with the air in the fuel air mixture from the fuel injector, and one stream used to dilute the hot combustion product to reduce the temperature to a value compatible with the working range of the turbine.
Gas turbine engines for aircraft are required to operate over a wide range of conditions which involve differing ratios of the mass flows of the combustion and dilution air streams. To ensure a high combustion efficiency it is usual for the proportion of the total airflow supplied to the burning zone to be determined by the amount of fuel required to be burned to produce the necessary heat input to the turbine at the cruise condition. An ideal air/fuel mixture ratio at cruise usually leads to an over rich mixture in the burning zone at high power conditions (such as take-off) with resultant soot and smoke emission. It is possible to reduce smoke emission at take-off by weakening the burning zone mixture strength but this involves an increase in primary zone air flow which reduces stability and makes ignition of the engine difficult to achieve, especially at altitude.
The temperature rise of the air in the combustor will depend on the amount of fuel burnt. Since the gas temperature required at the turbine varies according to the operating condition, the combustor must be capable of maintaining sufficient burn over a range of operating conditions. Unwanted emissions rise with increase in temperature and therefore it is desirable to keep the temperature low to reduce emissions of oxides of nitrogen. With increasingly stringent emission legislation, combustion temperature is an increasingly important factor and it is necessary that the combustor operates at temperatures of less than 2100K. However at low temperatures, the efficiency of the overall cycle is reduced.
It is a requirement for commercial airliners to decelerate rapidly in the case of potential collisions. In order to decelerate a gas turbine from high power to low power, the fuel flow to the engine must be reduced. Although the reduction in fuel flow is almost instantaneous, the rate of reduction of engine airflow is relatively slow because of the inertia of rotating parts such as turbines, compressors, shafts etc. This is produces a weak mixture of fuel and this increases the risk of flame extinction, especially at altitude. It is not always easy to relight the flame especially when the combustor is set to run weakly. Because modern combustors invariably operate in lean burn principles to reduce oxide of nitrogen emissions, combustors need to be operated as close to the lean extinction limit at all engine operating conditions. If margins are set wide enough to prevent flame extinction, emissions performance is compromised.
Combustion is initiated and stabilises in the pilot zone, the most upstream section of the combustor. Low power stability requires rich areas within the primary zone of the combustor, enabling combustion to occur when the overall air/fuel ratio is much weaker than the flammability limit of kerosene.
Conventional gas turbine engines are thus designed as a compromise rather than being optimised because of the above mentioned conflicting requirements at different operating conditions. New staged design of combustors have overcome these problems to a limited extent. These comprise two combustion zones (pilot and a main zones) each having a separate fuel supply. Essentially this type of combustor is designed such that a fixed flow of about 70% enters the combustor at the main zone and the remaining 30% of the air flows to the pilot zone. In such systems the air/fuel ratio is determined by selecting the amount of fuel in each stage, allowing greater control. U.S. Pat. No. 3,593,518 describes a combustion chamber having additional air inlets whicn can be controlled to vary proportions of airflow at various points. Current gas turbine engine trends are towards increased thrust/weight ratios which require the engine to perform at higher operating compression ratios and wider ranges of combustor air/fuel ratios. Future gas turbine combustion systems will be expected to perform at higher inlet temperatures and richer air/fuel ratios at high power. Because there is little variability in the airflows supplied to each zone, the amount of optimisation achievable for each operating condition is reduced. These combustor designs will also suffer from either high nitrogen oxide and/or smoke emissions at full power, or poor stability at low power.
It is therefore a requirement to improve control of the amount of fuel, air and air/fuel ratio entering the combustion zone which reduces the problems of weak flame extinction, emissions of oxides of nitrogen and unburnt fuel, whilst maintaining good efficiency and performance at all operating conditions. US Patent describes a fuel injector for an internal combustion chamber having fluid control means to vary the resistance of flow of exhaust. It is known requirement therefore to provide a fuel injector capable of varying the airflow into the combustor pilot zone. At high power, lower airflow is required to the pilot zone and the air fuel ratio should be set to avoid fuel rich zones and emissions at high power. Improved control of the primary zone air/fuel ratio and droplet sizes will allow a maximum flame speed to be achieved which will be hard to blow out, resulting in improved stability. The airflow within the primary zone of the combustor should be controllable and be able to be varied according to the power setting. It is known to control the degree of restriction experienced by air flow through the injector such that for a set upstream pressure the amount of air (and fuel) flow through the fuel injector can be varied. In addition this would also have an effect on the flow proportions of air which flows through the other combustor inlet ports. Varying the airflow into the primary zone through the fuel injector, will also effect atomisation quality. At idle, with airblast atomiser fuel injectors, low airflow results in low air velocity through the injector. The fuel atomisation process relies on the fast moving air flowing across the sheet of liquid fuel at higher power condition; higher airflow velocity through the fuel injector would promote good atomisation, fine droplets and low emissions. Thus modulating the airflow through the fuel injector (the largest contribution to airflow into the primary zone in modern combustion systems), would improve stability and reduce high power emissions.
One known method of providing greater control of air flow and air/fuel ratio is to use fuel injectors having variable geometry which control the amount of air and fuel flow through the fuel injector. Variable geometry fuel injectors have moving parts whose position alters the fuel and air flow resistance. Such designs have not found favour as they are not robust. In the high temperature atmosphere of the combustor and due to the complex nature of fuel injectors, moving parts are unreliable. It is therefore impractical to use such devices in a working gas turbine engine.
It is an object of the invention to provide flow mixing control at the fuel injector stage which can vary the air flow (or fuel) in a reliable and controllable manner.
It is an object of the invention to provide flow mixing control at the fuel injector stage which can vary the air flow (or fuel) in a reliable and controllable manner.
According to the invention is provided a fuel injector including a combustion air flow conduit, a fuel inlet, means to mix the air and fuel flowing therethrough, and fluidic control means including at least one control port, such that variation of flow of control air through said control port allows variation in the degree of flow resistance to which combustion air is subjected.
The advantage of such:a design of fuel injector is that it does not require moving parts and as such is inherently robust.
Preferably a fluid diverter is incorporated which diverts combustion air to either a first flow channel or a second flow channel each subjecting the flow to a varying degree of resistance. In a fluid diverter, the combustion air flow conduit divides into a first and second sub-conduit, said fluid control means comprising at least one port located adjacent to the confluence thus formed, such that selective over-pressure or under-pressure to the control port sets up a control flow therethrough, thereby selectively diverting the main flow to either the first or second sub-conduits, each sub-conduit subjecting combustion air to different degrees of flow resistance.
A typical modern fuel injector includes a number of swirlers. The swirling flow from the injector is required to form aerodynamic recirculation. Varying the swirl will vary the strength of the recirculation zones within the combustor, thus varying flow resistance. Preferably the fluidic control means allows variation in the degree of swirl to be varied.